As one can ascertain, the prior art is replete with pressure transducers or sensors employed in harsh environments. Such environments include deleterious substances which may destroy the transducer, as well as high pressures and temperatures. High temperatures include those temperatures which are found in various high temperature environments as combustion engines, for example. In other applications, such as the use of pressure transducers in injection molding and for other environments extremely high temperatures are also found. The prior art has disclosed pressure transducers which are capable of operating at very high temperatures as temperatures in excess of 600.degree. C. See for example, U.S. Pat. No. 7,124,639 which issued on Oct. 24, 2006, entitled “Ultra High Temperature Hermetically Protected Wirebonded Piezoresistive Transducer” by A. D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc., the assignee herein. See also U.S. Pat. No. 6,363,792 entitled “Ultra High Temperature Transducer Structure” issued on Apr. 2, 2002 to A. D. Kurtz et al. and assigned to the assignee herein. See also U.S. Pat. No. 6,530,282 entitled “Ultra High Temperature Transducer Structure” issued on Mar. 11, 2003 to A. D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc., the assignee herein.
By referring to the above noted patents, one can see applications of such transducers in high temperature environments as well as the monitoring of such signals in such environments. One problem is found when one deals in the oil and geothermal exploration fields. In such oil and geothermal explorations, one uses pressure or temperature transducers which are exposed to temperatures much higher than those experienced by standard electronics. Typical transducers which are used for normal operations are usually limited to temperatures below +125.degree. C. Due to the depth of drilling as well as the use of steam to extract the oil the operating temperature in such explorations exceed 200° C. Pressure transducers using a piezoresistive silicon-on-insulator (SOI) structure are widely used in such applications. Such transducers for example are described in the above noted patents. Also used are platinum resistors (RTD) used to measure the temperature which resistors are also capable of operating at these high temperatures. Thus, the combination afforded in regard to the above is that one requires a pressure transducer which can operate at high temperatures and one also requires electronics which can operate at such temperatures. See for example a co-pending application entitled “High Temperature Pressure Sensing System” Ser. No. 11/234,724 filed on Sep. 23, 2005 for A. D. Kurtz et al. and is assigned to the assignee herein. In that application, there is described a high temperature pressure sensing system which includes a transducer having pressure sensing piezoresistive elements formed by a SOI process. The system also uses SOI CMOS electronic circuitry which is operatively coupled to the piezoresistive sensor as well as ancillary circuitry connected to the unit to provide compensation and normalization. That application is incorporated by reference in its entirety herein.
Other examples of SOI technology may be seen in U.S. Pat. No. 5,955,771 entitled “Sensor for Use in High Vibrational Applications and Methods of Fabricating the Same” issued to A. D. Kurtz and U.S. Pat. No. 4,672,354.
In existing oil and geothermal applications, due to the depth of the drilling as well as due to the use of steam to extract the oil, very high temperatures are involved. In oil and geothermal explorations the wires used in these systems are extremely long and can be as long as 10,000 meters. These wires apart from being extremely long are also expensive. The cost of the wire often exceeds the cost of the transducers. In prior art applications, the pressure transducers are connected to the wiring via a four-to-twenty milliamp electronic interface. The second wire is the metal conduit in which the wire is inserted. The prior art method has significant temperature limitations which are further aggravated by the significant power dissipation of the four-to-twenty milliamp interface. This power dissipation increases the junction temperature of the electronics by several tens of degrees above ambient temperature. The prior art method also requires a separate wire for each pressure or temperature sensor.
The present invention discloses a way of interfacing one or more pressure sensors to a measuring system using only one wire for the signal and power and a return wire which is usually the conduit of the signal/power wire. An electronic interface is advantageous for sensors located in a very high temperature environment at great distances from the measuring system such as described above in the oil and geothermal explorations. The invention is also well suited for integration in a circuit using technology suitable for high temperature operation as the silicon-on-insulator (SOI) process. The signal transmission system or wire interface described is also applicable and advantageous for use in systems operating at more benign temperatures and over shorter distances as it simplifies the wiring as well as the measuring method.